Regulatory controllers are designed to adjust a variable of a plant (e.g., a device or process) automatically to conform a parameter of the plant (e.g., temperature, pressure, fluid level, etc.) to a particular value (referred to herein as the “set-point”) based on feedback for the parameter. For example, a regulatory controller may automatically adjust a control signal value supplied to a valve controlling fluid flow (the “variable”) into a tank to maintain the fluid level (the “parameter”) of the tank at a particular level (the “set-point”) based on fluid level feedback from the tank.
The performance of a regulatory controller is typically determined by its robustness, set-point tracking, and disturbance rejection. Robustness is the ability of the controller to remain stable when an actual parameter deviates from a model on which the regulatory controller is based. Set-point tracking is the ability of a regulatory controller to cause a parameter to follow a specified trajectory between a current set-point and a new set-point. Disturbance rejection is the ability of a regulatory controller to counteract the effects of external disturbances that would otherwise cause a parameter to deviate from a desired set-point. In addition, the performance of a regulatory controller is often determined based on its aggressiveness, which is the degree of vigorousness with which the regulatory controller attempts to maintain a parameter at a particular level.
Proportional, integral, derivative (PID) controllers, which are relatively inexpensive, are the most common type of regulatory controller used in process control applications. It is difficult to tune a PID controller, however, to meet robustness, set-point tracking, and disturbance rejection objectives because tuning parameter for PID controllers are not directly tied to individual ones of these objectives. The tuning parameters used in PID controllers each affect two or more of the typical objectives used to measure regulatory controller performance, which makes it difficult, if not impossible, to achieve certain performance objectives with PID controllers. For example, a PID controller cannot be tuned to achieve good set-point tracking and disturbance rejection simultaneously.
Other controllers such as fuzzy logic controllers, general linear controllers, state feedback controllers, and observer controllers are able to achieve better results than PID controller. Compared to PID controllers, however, these controllers are relatively complex and expensive.
There is an unmet need for inexpensive regulatory controllers that are easily tuned for robustness, set-point tracking, and disturbance rejection. The present invention addresses this need among others.